This invention relates to encoders for converting translational or rotational mechanical movement into electronic position signals. More particularly, this invention relates to analog-to-digital encoders for converting the physical position of a liquid level measuring instrument into digitally encoded electronic position signals.
Devices are known for measuring the level of a liquid confined in a reservoir, e.g. water contained by a dam, petroleum liquid in a storage tank, and wine contained in a vat. At present, there are two popular types of commercially available tank gauge systems used for this purpose, both of which employ a hollow conduit enclosing a perforated flexible tape translatably mounted in communication with the interior of the tank through the top at one end. The end of the tape interior to the tank is provided with 2 float member which follows the level of the liquid in the tank. The other end of the tape is attached to a spring motor or negator spring which maintains the tape taut so that motion of the float member is converted to translational motion of the tape and rotational motion of the sprockets or pulleys. In one of the commercially available devices, the tape is physically marked with a graduated scale and a window is provided in the conduit at eye level with a hairline or other sight level to enable readings to be taken. In the other commercially available unit, a reversible rotary mechanical counter having numbered indicator wheels is driven by one of the sprocket wheels and is mounted in a visibly accessible location.
In installations where there is poor lighting, e.g. in wine vats stored in a warehouse, or when it becomes necessary to take night time readings, it is typically difficult to obtain accurate readings with either type of gauge. A more serious limitation however, attendant with the use of commercially available gauges is the requirement that each reading must be obtained at a particular tank site which requires the presence of a human operator at that site. Thus, to obtain level measurements from a plurality of scattered petroleum storage tanks, one or more workmen must be dispatched to the individual tank sites. Similarly, when transferring stored liquids from one tank to another or among several tanks, individual workmen must be stationed at each tank site and a communication link must be provided among all workmen to enable the liquid flow to be monitored.
Other liquid level measuring devices are known which attempt to provide an automated indication of liquid levels, either in the form of an illuminated digital display, a permanent printed chart, a perforated card or the like, and some of which provide a local readout at the tank site, a central readout at a remote location or both. The following U.S. Pats. are believed to be representative of devices of this type: Nos. 2,112,371, 2,132,213, 2,192,421, 3,034,217, and 3,078,715. Such devices typically employ the conduit, tape and float mechanism generally described above, and in addition a variable resistance element, rotating drums with commutator segments and wiper brushes, or other elements which follow the motion of the tape and float and convert this motion to analog or digital electrical position signals. In the '217 patent noted above, a coded photographic film is translated past an optical encoder station in response to the movement of the perforated measuring tape to provide bilevel signals which are coupled to appropriate electromechanical circuitry for providing modified electrical signals indicative of the liquid level.
Encoder devices of which the above patents are representative suffer from several disadvantages. Both the analog and digital devices require costly, relatively sophisticated electromechanical subassemblies which are not readily compatible with existing tank liquid level gauges of the conduit and perforated tape variety, and thus necessitate expensive redesign to existing installations in order to be useful. The digital variety requires initial manual sequencing in order to obtain subsequent accurate readings and must be resequenced in the event of a temporary power failure. In installations having a plurality of devices of the latter type, each device must be manually reset, which renders their implementation even more undesirable. Further, those devices using a rotating drum with commutator segments and cooperating brushes must be periodically serviced by replacing the brushes and eventually the commutator segments due to mechanical wear and the collection of non-electrically conductive contaminants on the brush and commutator surfaces. To date, efforts to provide an absolute encoder for use with existing liquid level measuring gauges or similar devices, and free of the above disadvantages, have not been successful.